Shocking: Modern slaves -The real price of a cup of Tetley tea

Tens of thousands of girls as young as 12 are trafficked from India's remote north east every year. Many are the daughters of tea estate workers, and end up working as servants in the Indian capital New Delhi or trafficked on to the Middle East and the UK. Many suffer physical and sexual abuse at the hands of employers and few receive the wages they were promised.

Millions of Brits drink a cup of Assam tea each day, but it comes at a terrible price. Plantation workers on 12p an hour are easy prey for traffickers who lure away their daughters to India's cities. Now pressure is growing on big tea brands to safeguard better pay.

When the trafficker came knocking on the door of Elaina Kujar's hut on atea plantation at the north-eastern end of Assam, she had just got back from school. Elaina was 14 and wanted to be a nurse. Instead, she was about to lose four years of her life as a child slave.

She sits on a low chair inside the hut, playing with her long dark hair as she recalls how her owner would sit next to her watching porn in the living room of his Delhi house, while she waited to sleep on the floor. "Then he raped me," she says, looking down at her hands, then out of the door. Outside, the monsoon rain is falling on the tin roof and against the mud-rendered bamboo strip walls, on which her parents have pinned a church calendar bearing the slogan The Lord is Good to All.

Elaina was in that Delhi house for one reason: her parents, who picked the world-famous Assam tea on an estate in Lakhimpur district, were paid so little they could not afford to keep her. There are thousands like her, taken to Delhi from the tea plantations in the north-east Indian state by a trafficker, sold to an agent for as little as £45, sold on again to an employer for up to £650, then kept as slaves, raped, abused. It is a 21st-century slave trade. There are thought to be 100,000 girls as young as 12 under lock and key in Delhi alone: others are sold on to the Middle East and some are even thought to have reached the UK.

Every tea plantation pays the same wages. Every leaf of every box of Assam tea sold by Tetley and Lipton and Twinings and the supermarket own brands – Asda, Waitrose, Tesco, Sainsbury's and the rest – is picked by workers who earn a basic 12p an hour.

If it says Fairtrade on the box, or certified by the Rainforest Alliance or the Ethical Tea Partnership, it makes no difference: the worker received the same basic cash payment – 89 rupees (£1) a day, a little over half the legal wage for an unskilled worker in Assam of 158.54 rupees. To place that in context, a worker receives about 2p in cash for picking enough tea to fill a box of 80 tea bags, which then sells for upwards of £2 in the UK. The companies say they know the wages are low, and they are trying to make things better, but their hands are tied by the growers. The growers, who set the wages by collective bargaining, say it is all they can afford.

But there is a price for keeping wages so low, and it is paid by the workers who cannot afford to keep their daughters. When the traffickers come knocking, offering to take the girls away, promising good wages and an exciting new life, they find it hard to say no. "He said he would change our lives," says Elaina, now 20. "The tea garden was closed when he came and my parents were not working, so my father wanted to send me."

The trafficker had promised excitement and glamour: instead she started work every day at 4am and worked until midnight, and though he promised to give her 1,500 rupees a month, she was never paid. He kept her as a prisoner, unable to leave the house or contact her family.

"His wife was suspicious about what was happening. I told her he had raped me but he denied it and told me to shut up my mouth," she says. "After that, I was always crying, but he kept me locked in the house. I was afraid. I had no money and he threatened that I would end up in a brothel."

She was saved only when he sent her to a new owner who, on learning her story, sent her home.

Elaina's is not an isolated case. Rabina Khatun, now 18, discovered she had been sold into slavery when she agreed to go to Delhi to work as a maid. A woman from the village had tempted her with the promise of 3,000 rupees a month. "She said, 'Come and see Delhi. It is bigger than your village'," she says. She was 14: it was two years before she was allowed to go home. When she complained she had not been paid, she was sold on again to three men as a plaything. "I was taken to a house and they locked me in. Then they raped me. Afterwards they took me to Old Delhi station and left me there with no money. I was physically and mentally ill from what had happened to me. I want the men to be punished. I am never going to Delhi again. I am very angry. I want to kill them."

Both Elaina and Rabina gave the Observer written permission to identify them as a way of exposing the trade.

Indian government figures show 126,321 trafficked children were rescued from domestic service in 2011-12, a year-on-year increase of nearly 27%. But many anguished parents have no idea what has happened to their daughters. According to India's National Crime Record Bureau, a child goes missing in India every eight minutes, and more than a third are never found.

For the parents of the missing, the pain is hard to bear. Saphira Khatun carefully places the picture of her daughter Minu Begum on the table in front of her. There are tears in her eyes. Minu had been doing well in school and wanted to join the police. But her head was turned by promises of money from a female trafficker in the village. "She had big dreams," says her sister Munu, 20. "Any 12-year-old wants to go to the big city: it is more exciting than the village."

One evening, Minu failed to come home. Her family have not seen her since that day four years ago. "Nobody does anything to stop bad things happening to poor girls," says her 17-year-old sister Nadira. "Please help us to get our sister back here. Wherever she is in India, please give me my sister back."

Arjun and Mukti Tati's daughter Binita would be 17 now. She was 14 when the trafficker took her away with promises of money and a better life. But a year passed, then another, and no word came. They went to plead with the trafficker to help find her, but he refused. "She was a very gentle girl, always playing, very happy," Mukti says. "We went to him 100 times but he always said he had no information."

The traffickers live openly among the other villagers. They argue that they are victims too. Shobaha Tirki, 50, worked in the tea gardens for years, rarely earning more than 500 rupees a month. One day he met a big trafficker who promised him good money if he would send girls from the village to his placement agency. "I took maybe 20 girls from here to placement agencies in Delhi. A lot of them came back but five or six did not." He gets 10,000 rupees per girl. It is not hard to convince them to go with him, he says. "I tell them about Delhi and how it is good to go to a big city," he says. "I tell them they will have a room of their own and a bathroom of their own."

The girls who come back and have been cheated of their wages head straight for his house, he says. He tells them to talk to the agency. "They never get their money though," he says.

Many of the traffickers are women, who find it easier to convince the girls to go with them. Kusma Takri, 27, gets 4,000 rupees for every girl. She says she needs the money. "This is my job. I know the Delhi placement agencies are bad but I am caught between the placement agencies and poverty."

Rama Shankar Chaurasia, chair of child rights group Bachpan Bachao Andolan, says the scale of the trafficking is immense. "They are kept as slaves, their wages are withheld and taken by their placement agency or supplier, their employers are told not to pay them directly because if they do the girls will run away."

The going price for a maid can be as much as 60,000 rupees, he says. "The person who pays that feels they have purchased the girl. But the rising demand from the ever-growing middle class must not be at the cost of slavery for hundreds of thousands of children."

The UK brews 165 million cups of tea a day, importing about 10% of world production. Assam, in the far north-east of India, has more than 850 tea estates. Many of the workers are descendants of tribal groups from central India originally trafficked to Assam as labour during British rule.

The state produced 590 million kilos of tea last year, more than half of the total Indian production of 1 billion kg. It sells for 140 rupees a kilogram at auction in Assam, but according to a joint report by Oxfam and the Ethical Tea Partnership, workers there are paid at rates equivalent to just 40% of the average Indian wage.

According to the Indian Tea Association, all workers in the main Brahmaputra valley estates receive a basic cash wage of 89 rupees (£1) a day – a little over half the minimum legal wage. The ITA claims that benefits in kind make the total package worth 178 rupees a day for permanent workers and 158 rupees for temporary workers.

Its director general, Monojit Dasgupta, claims the tea plantations have an agreement with the state government that whatever its members pay will be regarded as the legal wage. "It is not a question of paying so little, it is what the industry can afford," he says. But the employers can achieve the basic legal minimum wage for the state only by including in the calculation statutory benefits such as maternity pay and sickness benefit, and discretionary items such as free tea.

Rights groups say this is unethical. "Tea planters and tea packers have known for years that wages in the industry are shockingly low – often below internationally acknowledged poverty levels," says Ron Oswald, general secretary of the International Union of Food Workers. "Yet instead of tackling this honestly they hide behind certification schemes and claims of additional benefits and food rations – which are in fact ways of keeping workers tied into semi-feudal relations with tea garden owners."

Murray Worthy, from War on Want, says benefits in kind are no substitute for the cash wages the workers deserve. "It beggars belief that the giant tea brands can justify these poverty wages. With the lion's share of the price of tea ending up in their profits, they could easily afford to ensure all tea workers are paid a decent wage".

There is a general agreement among the brands, retailers and certification bodies that wages on the tea plantations are a problem.

Fairtrade – which certifies Sainsbury's, Tesco and Waitrose teas – says it accepts that wage levels in Assam are "well below" living wage levels and says it is working with other certification bodies to improve them. A spokeswoman said that prices need to rise if progress is to be made on improving wages.

The Ethical Tea Partnership – which certifies Twinings and Tetley – says its members are powerless to set wages but points to a joint report commissioned with Oxfam and published in May, which highlighted problems with low pay. Commenting on the report, Alison Woodhead from Oxfam said: "No matter how big and powerful, individual tea companies or certification organisations cannot tackle the deep-rooted and complex barriers to a living wage on their own. The best chance we have of eradicating poverty wages is for the whole industry – producers, governments, retailers, trade unions, companies and certification organisations – to work together to find a solution. We are delighted that that process has now started and we will continue to support its progress."

The Rainforest Alliance – which certifies Unilever brands, including Lipton – says it hopes certified farms will be paying a realistic living wage within five years.

Typhoo says it works closely with all three certification bodies to improve conditions on its plantations.

The Indian government hopes to have 335 anti-trafficking units set up in police stations around the country by the end of this year. But even the police admit there is no simple solution.

At Laluk police station, the faces of missing girls stare out from the noticeboard. Sub-inspector Nirmal Biswas, the newly arrived officer in charge, sits behind a large desk next to his crime chart. It lists 24 kidnappings for 2012, against 10 rapes and 17 thefts.

He reports that in the past month police have registered four cases of trafficking, and recovered one girl and her trafficker.

It is progress, he says. But it will not stop the trade, because the money which has been earmarked for the area by the government never reaches those who need it.

"It is the poverty here," he says. "If any trafficker offers 1,000 rupees, they will get girls. It will be defeated only with employment and development and eradication of poverty."